Goodbye, Mr. kilogram!

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1 Goodbye, Mr. kilogram! Amol Dighe Department of Theoretical Physics Tata Institute of Fundamental Research Chai-and-Why, Prithvi Theatre, Oct 1, 2017

2 Goodbye, Mr. kilogram! 1 Why? 2 How? 3 Why now? 4 What now? 5 How does it affect me?

3 Goodbye, Mr. kilogram! 1 Why? The retirement of Le Grand K 2 How? Lessons from the history of the standard metre 3 Why now? Necessary science and technology 4 What now? Redefining the other units 5 How does it affect me? Why all this is being done

4 The International Prototype Kilogram (IPK) A.k.a. Le Grand K Iridium-platinum cylinder d h 39 mm Kept in a vault near Paris Used to make replicas (very rarely)

5 Problems with a physical object as standard All eggs in one basket... A small accident would change everything! Inconvenient and undemocratic! Since 1889, the IPK / LGK could have changed by 50µg, in either direction! But what is the alternative?

6 Problems with a physical object as standard All eggs in one basket... A small accident would change everything! Inconvenient and undemocratic! Since 1889, the IPK / LGK could have changed by 50µg, in either direction! But what is the alternative?

7 Problems with a physical object as standard All eggs in one basket... A small accident would change everything! Inconvenient and undemocratic! Since 1889, the IPK / LGK could have changed by 50µg, in either direction! But what is the alternative?

8 Problems with a physical object as standard All eggs in one basket... A small accident would change everything! Inconvenient and undemocratic! Since 1889, the IPK / LGK could have changed by 50µg, in either direction! But what is the alternative?

9 The metrologists in charge CGPM: Conférence Générale des Poids et Mesures (General Conference on Weights and Measures) CIPM: Comité International des Poids et Mesures (International Committee for Weights and Measures) BIPM: Bureau International des Poids et Mesures (International Bureau for Weights and Measures) NIST: National Institute of Standards and Technology, USA Information and pictures from the websites of above organizations, and from Wikipedia Commons

10 Goodbye, Mr. kilogram! 1 Why? The retirement of Le Grand K 2 How? Lessons from the history of the standard metre 3 Why now? Necessary science and technology 4 What now? Redefining the other units 5 How does it affect me? Why all this is being done

11 Length units

12 The definitions of a metre Pendulum-based (pre-french revolution) Length of a seconds pendulum (half-period of a second) Varies widely from place to place (depends on gravity) Meridian-based (1795 ) 1/10,000,000 th of the half-meridian through Paris Metre de Archives (1799): accuracy of 0.1 mm

13 The definitions of a metre Pendulum-based (pre-french revolution) Length of a seconds pendulum (half-period of a second) Varies widely from place to place (depends on gravity) Meridian-based (1795 ) 1/10,000,000 th of the half-meridian through Paris Metre de Archives (1799): accuracy of 0.1 mm

14 The definitions of a metre Pendulum-based (pre-french revolution) Length of a seconds pendulum (half-period of a second) Varies widely from place to place (depends on gravity) Meridian-based (1795 ) 1/10,000,000 th of the half-meridian through Paris Metre de Archives (1799): accuracy of 0.1 mm

International prototype metre (1889 1960) Platinum-Iridium 0 temperature one

15 International prototype metre ( ) Platinum-Iridium 0 temperature one standard atmospheric pressure supported by 2 cylinders at 571 cm separation Accuracy: 2 µm

16 Quantum Mechanics makes things more accurate... Based on krypton line ( ) 1,650, wavelengths of light from the 2p 10 5d 5 atomic transition in krypton-86 Accuracy: 0.01 µm Kr line later found to be not-so-clean when compared with more accurate lines from lasers

17 Quantum Mechanics makes things more accurate... Based on krypton line ( ) 1,650, wavelengths of light from the 2p 10 5d 5 atomic transition in krypton-86 Accuracy: 0.01 µm Kr line later found to be not-so-clean when compared with more accurate lines from lasers

73 wavelengths of light from the 2p 10 5d 5 atomic transition in

18 Quantum Mechanics makes things more accurate... Based on krypton line ( ) 1,650, wavelengths of light from the 2p 10 5d 5 atomic transition in krypton-86 Accuracy: 0.01 µm Kr line later found to be not-so-clean when compared with more accurate lines from lasers

19 Special Relativity relates distance and time... Based on the speed of light (1983 ) Distance travelled by light in a vacuum in 1/299,792,458 th of a second Back to distance in terms of time? (remember pendulum) This is different! No dependence on source Special Relativity checked by multitude of experiments Speed of light in vacuum is a fundamental constant of nature that is independent of everything else! A second can be determined with an accuracy of better than 1 in a trillion

20 Special Relativity relates distance and time... Based on the speed of light (1983 ) Distance travelled by light in a vacuum in 1/299,792,458 th of a second Back to distance in terms of time? (remember pendulum) This is different! No dependence on source Special Relativity checked by multitude of experiments Speed of light in vacuum is a fundamental constant of nature that is independent of everything else! A second can be determined with an accuracy of better than 1 in a trillion

21 Special Relativity relates distance and time... Based on the speed of light (1983 ) Distance travelled by light in a vacuum in 1/299,792,458 th of a second Back to distance in terms of time? (remember pendulum) This is different! No dependence on source Special Relativity checked by multitude of experiments Speed of light in vacuum is a fundamental constant of nature that is independent of everything else! A second can be determined with an accuracy of better than 1 in a trillion

22 Defining one second Obsolete: day-based 1/86400 th of a mean day Atomic transition-based 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom At the mean sea level (Why?) General relativity!

23 Defining one second Obsolete: day-based 1/86400 th of a mean day Atomic transition-based 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom At the mean sea level (Why?) General relativity!

24 Defining one second Obsolete: day-based 1/86400 th of a mean day Atomic transition-based 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom At the mean sea level (Why?) General relativity!

25 Defining one second Obsolete: day-based 1/86400 th of a mean day Atomic transition-based 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom At the mean sea level (Why?) General relativity!

26 Defining one second Obsolete: day-based 1/86400 th of a mean day Atomic transition-based 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom At the mean sea level (Why?) General relativity!

27 The principle behind accurate definition of one metre Choose definition of time based on a very accurate measurement Choose a fundamental constant of nature that connects time and distance Measure the fundamental constant as accurately as possible, and freeze its value Through this fundamental constant, define one metre This definition will now be everlasting...

28 The principle behind accurate definition of one metre Choose definition of time based on a very accurate measurement Choose a fundamental constant of nature that connects time and distance Measure the fundamental constant as accurately as possible, and freeze its value Through this fundamental constant, define one metre This definition will now be everlasting...

29 The principle behind accurate definition of one metre Choose definition of time based on a very accurate measurement Choose a fundamental constant of nature that connects time and distance Measure the fundamental constant as accurately as possible, and freeze its value Through this fundamental constant, define one metre This definition will now be everlasting...

30 The principle behind accurate definition of one metre Choose definition of time based on a very accurate measurement Choose a fundamental constant of nature that connects time and distance Measure the fundamental constant as accurately as possible, and freeze its value Through this fundamental constant, define one metre This definition will now be everlasting...

31 The principle behind accurate definition of one metre Choose definition of time based on a very accurate measurement Choose a fundamental constant of nature that connects time and distance Measure the fundamental constant as accurately as possible, and freeze its value Through this fundamental constant, define one metre This definition will now be everlasting...

32 Back to kilogram Fundamental constants needed Fundamental constants should connect mass with distance and/or time Planck s constant connects energy and time E = h f Special relativity connects rest mass and energy E = mc 2 m = h f /c 2 A combination of Planck s constant and speed of light OK Definition of a kilogram Planck s constant h = J-s One kg is that mass, whose rest-mass energy would be equal to that of light with frequency 1 f = Hz

33 Back to kilogram Fundamental constants needed Fundamental constants should connect mass with distance and/or time Planck s constant connects energy and time E = h f Special relativity connects rest mass and energy E = mc 2 m = h f /c 2 A combination of Planck s constant and speed of light OK Definition of a kilogram Planck s constant h = J-s One kg is that mass, whose rest-mass energy would be equal to that of light with frequency 1 f = Hz

34 Back to kilogram Fundamental constants needed Fundamental constants should connect mass with distance and/or time Planck s constant connects energy and time E = h f Special relativity connects rest mass and energy E = mc 2 m = h f /c 2 A combination of Planck s constant and speed of light OK Definition of a kilogram Planck s constant h = J-s One kg is that mass, whose rest-mass energy would be equal to that of light with frequency 1 f = Hz

35 Back to kilogram Fundamental constants needed Fundamental constants should connect mass with distance and/or time Planck s constant connects energy and time E = h f Special relativity connects rest mass and energy E = mc 2 m = h f /c 2 A combination of Planck s constant and speed of light OK Definition of a kilogram Planck s constant h = J-s One kg is that mass, whose rest-mass energy would be equal to that of light with frequency 1 f = Hz

36 Goodbye, Mr. kilogram! 1 Why? The retirement of Le Grand K 2 How? Lessons from the history of the standard metre 3 Why now? Necessary science and technology 4 What now? Redefining the other units 5 How does it affect me? Why all this is being done

37 Could we have done this years ago? No... Special relativity just getting established No Quantum mechanics General relativity just proposed 50 years ago? 25 years ago? 10 years ago? Let us see what technology is needed to measure h

38 Could we have done this years ago? No... Special relativity just getting established No Quantum mechanics General relativity just proposed 50 years ago? 25 years ago? 10 years ago? Let us see what technology is needed to measure h

39 Could we have done this years ago? No... Special relativity just getting established No Quantum mechanics General relativity just proposed 50 years ago? 25 years ago? 10 years ago? Let us see what technology is needed to measure h

40 Could we have done this years ago? No... Special relativity just getting established No Quantum mechanics General relativity just proposed 50 years ago? 25 years ago? 10 years ago? Let us see what technology is needed to measure h

41 The Watt balance technique for h Watt balance movie Mechanical vs. electrical forces, using QM phenomena Current measured using Josephson junction (discovered 1962, Nobel 1973), Josephson constant K J = 2e/h Voltage measured using fractional quantum Hall effect (discovered van Kiltzing 1980, Nobel Laughlin-Stormet-Tsui 1998), von Kiltzing constant R K = h/e 2 Fundamental discoveries in QM were needed!

42 The Watt balance technique for h Watt balance movie Mechanical vs. electrical forces, using QM phenomena Current measured using Josephson junction (discovered 1962, Nobel 1973), Josephson constant K J = 2e/h Voltage measured using fractional quantum Hall effect (discovered van Kiltzing 1980, Nobel Laughlin-Stormet-Tsui 1998), von Kiltzing constant R K = h/e 2 Fundamental discoveries in QM were needed!

43 Progress in measurement of h

44 Confirmatory test through another constant, N A The Avogadro project A pure silicon-28 sphere, Diameter 94 mm Exactly spherical, accurate to 1 nanometer Roundness confirmed by interference, crystal structure by X-ray diffraction Measure mass, calculate number of atoms Avogadro s constant N A one kilogram (equal to 1/12th mass of N A C-12 atoms) h

$by interference, crystal structure by X-ray diffraction Measure mass, calculate number of atoms Avogadro s constant N A one kilogram (equal to 1/12th mass of N A C-12 atoms)$

45 Confirmatory test through another constant, N A The Avogadro project A pure silicon-28 sphere, Diameter 94 mm Exactly spherical, accurate to 1 nanometer Roundness confirmed by interference, crystal structure by X-ray diffraction Measure mass, calculate number of atoms Avogadro s constant N A one kilogram (equal to 1/12th mass of N A C-12 atoms) h

46 Self-imposed conditions to be fulfilled (CGPM) At least three independent experiments, including both watt balance and Avogadro project, should yield an accuracy of at least 50 parts in a billion. At least one of these results should have an uncertainty not larger than 20 parts in a billion All experiments should be consistent at 95% level of confidence. All conditions have now been fulfilled!! (July 2017)

47 Self-imposed conditions to be fulfilled (CGPM) At least three independent experiments, including both watt balance and Avogadro project, should yield an accuracy of at least 50 parts in a billion. At least one of these results should have an uncertainty not larger than 20 parts in a billion All experiments should be consistent at 95% level of confidence. All conditions have now been fulfilled!! (July 2017)

48 Goodbye, Mr. kilogram! 1 Why? The retirement of Le Grand K 2 How? Lessons from the history of the standard metre 3 Why now? Necessary science and technology 4 What now? Redefining the other units 5 How does it affect me? Why all this is being done

49 The other SI units

50 Redefining one ampere (A) One ampere now That constant current which, if maintained in two straight parallel conductors of infinite length and negligible circular cross-section and placed 1 m apart in vacuum, would produce a force per unit length F/l = newton per metre The new one ampere That amount of current such that the electron charge e = ampere-second

51 Redefining one ampere (A) One ampere now That constant current which, if maintained in two straight parallel conductors of infinite length and negligible circular cross-section and placed 1 m apart in vacuum, would produce a force per unit length F/l = newton per metre The new one ampere That amount of current such that the electron charge e = ampere-second

52 Redefining one mole (mol) One mole now The amount of substance of a system that contains as many elementary entities as there are atoms in kilogram of carbon-12 The new one mole The amount of substance that has number of units equal to Avogadro s constant N A =

53 Redefining one mole (mol) One mole now The amount of substance of a system that contains as many elementary entities as there are atoms in kilogram of carbon-12 The new one mole The amount of substance that has number of units equal to Avogadro s constant N A =

54 Redefining one kelvin (K ) One kelvin now The fraction 1/ of the thermodynamic temperature of the triple point of water The new one kelvin One K is defined such that the Boltzmann constant k B = joules/kelvin The principle Thermodynamics: E = (3/2)k B T for monoatomic gases: temperature is a measure of energy.

55 Redefining one kelvin (K ) One kelvin now The fraction 1/ of the thermodynamic temperature of the triple point of water The new one kelvin One K is defined such that the Boltzmann constant k B = joules/kelvin The principle Thermodynamics: E = (3/2)k B T for monoatomic gases: temperature is a measure of energy.

56 Redefining one kelvin (K ) One kelvin now The fraction 1/ of the thermodynamic temperature of the triple point of water The new one kelvin One K is defined such that the Boltzmann constant k B = joules/kelvin The principle Thermodynamics: E = (3/2)k B T for monoatomic gases: temperature is a measure of energy.

57 Rewording candela (cd): unit of luminous intensity The old candela The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency Hz, and that has a radiant intensity in that direction of 1/683 watt per steradian The re-worded candela One candela is that unit in which the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency Hz is K cd = 683 candela-steradian per watt

Rewording candela (cd): unit of luminous intensity The old candela The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 10 12 Hz, and that has

58 Rewording candela (cd): unit of luminous intensity The old candela The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency Hz, and that has a radiant intensity in that direction of 1/683 watt per steradian The re-worded candela One candela is that unit in which the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency Hz is K cd = 683 candela-steradian per watt

59 The new SI system

60 Goodbye, Mr. kilogram! 1 Why? The retirement of Le Grand K 2 How? Lessons from the history of the standard metre 3 Why now? Necessary science and technology 4 What now? Redefining the other units 5 How does it affect me? Why all this is being done

61 Any practical changes now? Do I get heavier / taller / brighter? No! Nothing changes! But that s the whole idea! The units are a part of everyday life for everything, so they should not be disturbed. The values of units are not changed (as far as possible, to a few parts in a billion) That s why going to all this trouble of fixing the value of h We are fixing the units for posterity, so that even if no physical standards of today survive, people after centuries will still know what exactly we meant by a second / metre / kilogram / ampere...

62 Any practical changes now? Do I get heavier / taller / brighter? No! Nothing changes! But that s the whole idea! The units are a part of everyday life for everything, so they should not be disturbed. The values of units are not changed (as far as possible, to a few parts in a billion) That s why going to all this trouble of fixing the value of h We are fixing the units for posterity, so that even if no physical standards of today survive, people after centuries will still know what exactly we meant by a second / metre / kilogram / ampere...

63 Any practical changes now? Do I get heavier / taller / brighter? No! Nothing changes! But that s the whole idea! The units are a part of everyday life for everything, so they should not be disturbed. The values of units are not changed (as far as possible, to a few parts in a billion) That s why going to all this trouble of fixing the value of h We are fixing the units for posterity, so that even if no physical standards of today survive, people after centuries will still know what exactly we meant by a second / metre / kilogram / ampere...

64 Any practical changes now? Do I get heavier / taller / brighter? No! Nothing changes! But that s the whole idea! The units are a part of everyday life for everything, so they should not be disturbed. The values of units are not changed (as far as possible, to a few parts in a billion) That s why going to all this trouble of fixing the value of h We are fixing the units for posterity, so that even if no physical standards of today survive, people after centuries will still know what exactly we meant by a second / metre / kilogram / ampere...

65 Unification of measurement units We now need to make only one measurement: the frequency of Cs-133 oscillations This can be made by anyone, anywhere democracy of measurements Demo of a crystal with accurate time measurement The forever fixed fundamental constants allow us to reconstruct all other units It s a different way of thinking about units: All fundamental units are essentially the same!

66 Unification of measurement units We now need to make only one measurement: the frequency of Cs-133 oscillations This can be made by anyone, anywhere democracy of measurements Demo of a crystal with accurate time measurement The forever fixed fundamental constants allow us to reconstruct all other units It s a different way of thinking about units: All fundamental units are essentially the same!

67 Unification of measurement units We now need to make only one measurement: the frequency of Cs-133 oscillations This can be made by anyone, anywhere democracy of measurements Demo of a crystal with accurate time measurement The forever fixed fundamental constants allow us to reconstruct all other units It s a different way of thinking about units: All fundamental units are essentially the same!

68 Unification of measurement units We now need to make only one measurement: the frequency of Cs-133 oscillations This can be made by anyone, anywhere democracy of measurements Demo of a crystal with accurate time measurement The forever fixed fundamental constants allow us to reconstruct all other units It s a different way of thinking about units: All fundamental units are essentially the same!

69 Why these strange values of constants then? Speed of light c = 299, 792, 458 metres/second Planck s constant h = joules-second Electron charge e = ampere-second Avogadro s constant N A = units/mole Boltzmann s constant k B = joules/kelvin Luminous efficacy K cd = 683 candela-steradian per watt We did not know... When these units started being used, we did not know that they are actually connected by laws of nature

70 Why these strange values of constants then? Speed of light c = 299, 792, 458 metres/second Planck s constant h = joules-second Electron charge e = ampere-second Avogadro s constant N A = units/mole Boltzmann s constant k B = joules/kelvin Luminous efficacy K cd = 683 candela-steradian per watt We did not know... When these units started being used, we did not know that they are actually connected by laws of nature

71 Possible natural units (natunits?) Once a natsecond is defined: Natmetre: Speed of light c = 1 natmetre/ natsecond Natkilogram: Planck s constant h = 1 natjoules- natsecond Natampere: Electron charge e = 1 natampere-natsecond Natmole: Avogadro s constant N A = 1 units/natmole Natkelvin: Boltzmann s constant k B = 1 natjoules/natkelvin Natcandela: Luminous efficacy K cd = 1 natcandela-steradian per natwatt Physicists do this often... Natural system of units: c = 1, h = 2π, k B = 1 Do we use such units sometime?

72 Possible natural units (natunits?) Once a natsecond is defined: Natmetre: Speed of light c = 1 natmetre/ natsecond Natkilogram: Planck s constant h = 1 natjoules- natsecond Natampere: Electron charge e = 1 natampere-natsecond Natmole: Avogadro s constant N A = 1 units/natmole Natkelvin: Boltzmann s constant k B = 1 natjoules/natkelvin Natcandela: Luminous efficacy K cd = 1 natcandela-steradian per natwatt Physicists do this often... Natural system of units: c = 1, h = 2π, k B = 1 Do we use such units sometime?

73 Possible natural units (natunits?) Once a natsecond is defined: Natmetre: Speed of light c = 1 natmetre/ natsecond Natkilogram: Planck s constant h = 1 natjoules- natsecond Natampere: Electron charge e = 1 natampere-natsecond Natmole: Avogadro s constant N A = 1 units/natmole Natkelvin: Boltzmann s constant k B = 1 natjoules/natkelvin Natcandela: Luminous efficacy K cd = 1 natcandela-steradian per natwatt Physicists do this often... Natural system of units: c = 1, h = 2π, k B = 1 Do we use such units sometime?

74 Units many aspects of science second: General theory of relativity metre: Special theory of relativity kilogram: Quantum mechanics ampere: Atomic physics, Electrodynamics mol: Chemistry, Atomic-molecular physics kelvin: Thermodynamics, statistical physics candela: Optics we have been using this esoteric physics in our daily lives for things as simple as measuring a distance or a mass...

75 Units many aspects of science second: General theory of relativity metre: Special theory of relativity kilogram: Quantum mechanics ampere: Atomic physics, Electrodynamics mol: Chemistry, Atomic-molecular physics kelvin: Thermodynamics, statistical physics candela: Optics we have been using this esoteric physics in our daily lives for things as simple as measuring a distance or a mass...

76 SI system of units: old (now) vs. new New SI

77 What if the constants change? Sonuu, tulaa constants-var bharavsa naay kaay? Testing variation of fundamental constants Continuous laboratory testing (that s one job of the metrologists) Indirect evidences by observing the universe Fundamental constants are enshrined in theories like Relativity / QM / Thermodynamics. Any test of these theories is a test of the constancy of fundamental constants. But if some constant is indeed observed to change.. We ll have exciting times ahead...

78 What if the constants change? Sonuu, tulaa constants-var bharavsa naay kaay? Testing variation of fundamental constants Continuous laboratory testing (that s one job of the metrologists) Indirect evidences by observing the universe Fundamental constants are enshrined in theories like Relativity / QM / Thermodynamics. Any test of these theories is a test of the constancy of fundamental constants. But if some constant is indeed observed to change.. We ll have exciting times ahead...

79 What if the constants change? Sonuu, tulaa constants-var bharavsa naay kaay? Testing variation of fundamental constants Continuous laboratory testing (that s one job of the metrologists) Indirect evidences by observing the universe Fundamental constants are enshrined in theories like Relativity / QM / Thermodynamics. Any test of these theories is a test of the constancy of fundamental constants. But if some constant is indeed observed to change.. We ll have exciting times ahead...

80 What if the constants change? Sonuu, tulaa constants-var bharavsa naay kaay? Testing variation of fundamental constants Continuous laboratory testing (that s one job of the metrologists) Indirect evidences by observing the universe Fundamental constants are enshrined in theories like Relativity / QM / Thermodynamics. Any test of these theories is a test of the constancy of fundamental constants. But if some constant is indeed observed to change.. We ll have exciting times ahead...

81 What if the constants change? Sonuu, tulaa constants-var bharavsa naay kaay? Testing variation of fundamental constants Continuous laboratory testing (that s one job of the metrologists) Indirect evidences by observing the universe Fundamental constants are enshrined in theories like Relativity / QM / Thermodynamics. Any test of these theories is a test of the constancy of fundamental constants. But if some constant is indeed observed to change.. We ll have exciting times ahead...

82 What if the constants change? Sonuu, tulaa constants-var bharavsa naay kaay? Testing variation of fundamental constants Continuous laboratory testing (that s one job of the metrologists) Indirect evidences by observing the universe Fundamental constants are enshrined in theories like Relativity / QM / Thermodynamics. Any test of these theories is a test of the constancy of fundamental constants. But if some constant is indeed observed to change.. We ll have exciting times ahead...

Goodbye, Mr. kilogram!

Goodbye, Mr. kilogram! Amol Dighe Department of Theoretical Physics Tata Institute of Fundamental Research Chai-and-Why, Prithvi Theatre, Oct 1, 2017 Goodbye, Mr. kilogram! 1 Why? 2 How? 3 Why now? 4 What